Beryllium is a high strength, light weight, high stiffness metal that has extremely low ductility which prevents it from being cast and also creates a very low resistance to impact and fatigue, making the cast metal or metal produced from castings relatively useless for most applications.
To increase the ductility of beryllium, much work has been done with beryllium-aluminum alloys to make a ductile, two phase, composite of aluminum and beryllium. Aluminum does not react with the reactive beryllium, is ductile, and is relatively lightweight, making it a suitable candidate for improving the ductility of beryllium, while keeping the density low.
However, beryllium-aluminum alloys are inherently difficult to cast due to the mutual insolubility of beryllium and aluminum in the solid phase and the wide solidification temperature range typical in this alloy system. An alloy of 60 weight % beryllium and 40 weight % aluminum has a liquidus temperature (temperature at which solidification begins) of nearly 1250.degree. C. and a solidus temperature (temperature of complete solidification) of 645.degree. C. During the initial stages of solidification, primary beryllium dendrites form in the liquid to make a two phase solid-liquid mixture. The beryllium dendrites produce a tortuous channel for the liquid to flow and fill during the last stages of solidification. As a result, shrinkage cavities develop, and these alloys typically exhibit a large amount of microporosity in the as-cast condition. This feature greatly affects the properties and integrity of the casting. Porosity leads to low strength and premature failure at relatively low ductilities. In addition, castings have a relatively coarse microstructure of beryllium distributed in an aluminum matrix, and such coarse microstructures generally result in low strength and low ductility. To overcome the problems associated with cast structures, a powder metallurgical approach has been used to produce useful materials from beryllium-aluminum alloys.
There have also been proposed ternary beryllium-aluminum alloys made by powder metallurgical approaches such as liquid phase sintering. For example, U.S. Pat. No. 3,322,512, Krock et al., May 30, 1967, discloses a beryllium-aluminum-silver composite containing 50 to 85 weight % beryllium, 10.5 to 35 weight % aluminum, and 4.5 to 15 weight % silver. The composite is prepared by compacting a powder mixture having the desired composition, including a fluxing agent of alkali and alkaline earth halogenide agents such as lithium fluoride-lithium chloride, and then sintering the compact at a temperature below the 1277.degree. C. melting point of beryllium but above the 620.degree. C. melting point of the aluminum-silver alloy so that the aluminum-silver alloy liquifies and partially dissolves the small beryllium particles to envelope the brittle beryllium in a more ductile aluminum-silver-beryllium alloy. U.S. Pat. No. 3,438,751, issued to Krock et at. on Apr. 15, 1969, discloses a beryllium-aluminum-silicon composite containing 50 to 85 weight % beryllium, 13 to 50 weight % aluminum, and a trace to 6.6 weight % silicon, also made by the above-described powder metallurgical liquid sintering technique. However, high silicon content reduces ductility to unacceptably low levels, and high silver content increases alloy density. Therefore, the alloys cannot be successfully cast.
Other ternary, quaternary and more complex beryllium-aluminum alloys made by powder metallurgical approaches such as solid state synthesis have also been proposed. See, for example, McCarthy et at., U.S. Pat. No. 3,664,889. That patent discloses preparing the alloys by atomizing a binary beryllium-aluminum alloy to create a powder that then has mixed into it fine elemental metallic powders of the desired alloying elements. The powders are then mixed together thoroughly to achieve good distribution, and the powder blend is consolidated by a suitable hot or cold operation, carded on without any melting. These are not cast alloys and this approach is very costly.
It is known, however, that beryllium-aluminum alloys tend to separate or segregate when cast and generally have a porous cast structure. Accordingly, previous attempts to produce beryllium-aluminum alloys by casting resulted in low strength, low ductility, and coarse microstructures with poor internal quality.